Techniques for selectively collecting carbon dioxide from flue gas in thermal power plants include wet absorption, dry adsorption, membrane separation and cryogenics. Of these, wet absorption using an aqueous solution of monoethanolamine (MEA) is the most widely used and technically complete method. In accordance with this method, carbon dioxide in the flue gas is absorbed by reacting with the MEA aqueous solution diluted to 30% or less and then the solution is heated to separate a high concentration of carbon dioxide and regenerate the MEA. This wet absorption method is an aqueous solution-based process and thus has advantages of easy heat exchange and high carbon dioxide selectivity due to strong adsorption heat of amine. However, this method requires a large amount of energy for the regeneration process of the aqueous amine solution and has disadvantages such as loss by evaporation of small amine molecules and corrosion of equipment. In addition, this method is a very uneconomic technology in this point since cost and scale-up problems are very sensitive in the industrial sites where large-scale treatment of greenhouse gases is required. For this reason, amine-based dry adsorbents having high carbon dioxide selectivity while requiring less energy to regenerate the adsorbents are emerging as a new alternative.
Amine-based dry adsorbents absorb carbon dioxide through strong chemical bonding between carbon dioxide and amine, like wet adsorption using an aqueous amine solution. Adsorption capacity of such a dry adsorbent can be maximized by increasing the amount of amine supported using supports having high carbon dioxide selectivity and high porosity. As a representative example thereof, research results have been reported that the adsorption capacity of carbon dioxide can be dramatically improved by supporting an amine polymer as much as possible in a silica support having a very large pore volume (Zhang, H. et al., RSC Adv. 4, 2014, 19403-19417). A great deal of studies on such amine-based dry adsorbents has been focused on maximizing carbon dioxide adsorption performance by effectively carrying the maximum amine polymer through structural control of amine supports. However, in order to operate adsorbents for a long period of time in an actual process, it is necessary to additionally consider not only maximization of adsorption performance but also regeneration stability of the adsorbent, but studies on the regeneration stability are insufficient.
In fact, amine-based adsorbents are inactivated by various gases present in the flue gas. When the hydrothermal stability of the amine support is low, the performance of the adsorbents decreases due to structural deformation of the support when exposed to steam at a high temperature. In addition, it is known that, when the amine is exposed to high-temperature dried carbon dioxide during the regeneration process, it is rapidly inactivated due to production of urea and is also seriously inactivated by acid gas such oxygen or sulfur dioxide. Fortunately, solutions to most of the inactivation problems, except for the inactivation problem by acid gases, have recently been successfully suggested through the improvement of adsorbents by researchers from Korea and other countries. For example, the problem of urea production caused by reaction with carbon dioxide at high temperatures could be solved by applying an amine having a secondary amine structure to the adsorbent (Sayari, A. et al., Langmuir 28, 4241 (2012); Choi, W. et al., Nature Communications 7, 12640 (2016)), and the structural collapse of the support by steam could be solved by using a metal oxide having a very high hydrothermal stability as a support (Jones, C. W. et al., Applied Materials & Interfaces 2, 3363 (2010); Min, K. et al., ChemSusChem 10, 2518 (2017)). Unfortunately, however, problems associated with inactivity due to acidic gases such as oxygen and sulfur dioxide remain unsolved as a major hurdle to commercialization.
As mentioned above, the presence of oxygen which causes oxidative decomposition of amine at high temperatures is a major cause of rapid decrease in the stability of amine-based solid adsorbents during the continuous adsorption/desorption process. In an attempt to solve this phenomenon, relatively limited studies have been conducted to address the problem of amine oxidation. For example, research results were reported by Chuang and his colleagues at Akron University that the addition of polyethylene glycol (PEG) to silica adsorbents, in which polyethyleneimine is supported, can inhibit the oxidative degradation of amines through hydrogen bonding between amines and hydroxyl groups in the PEG molecule (Chuang, S. S. C. et al., ChemSusChem 5, 1435 (2012)). C. W. Jones group at Georgia Tech. reported research results that an adsorbent in which polypropyleneimine is supported rather than polyethyleneimine exhibits improved oxidation resistance as the distance between the amine groups increases (Jones, C. W. et al., Journal of the American Chemical Society 139, 3627 (2017)). However, such related art has a limitation in that the oxidative decomposition of amines cannot be remarkably improved to the level which is necessary for commercialization of adsorbents.
As described above, the oxidation resistance of solid amine adsorbents has not been actively studied. However, studies have been actively conducted to improve the oxidation resistance of aqueous amine solutions in case of a wet absorption method having a relatively long history of the technology. In case of wet adsorbents using MEA solutions, studies have been conducted on the introduction of various oxidation inhibitors to inhibit oxidative decomposition of amines (Rochelle, G. T. et al., Industrial & Engineering Chemistry Research 53, 16222 (2014)). Scavengers that directly remove radicals and activated oxygen, and chelating agents that can remove metal ions (Fe, Cu) functioning to catalyze radical formation with complex compounds have been studied as antioxidants. Most of these antioxidants have been found to have positive effects, but there are limitations on requiring continuous injection of antioxidants, since oxygen is continuously introduced under adsorption conditions and metal ions are continuously leached by corrosion in the reactor.
Since amine-based solid adsorbents have no reactor corrosion problem, the effects of metal impurities facilitating amine oxidation have been neglected. However, the present inventors have found that metal impurities are present in ppm in various amine compounds, and that the oxidation resistance of amines can be remarkably improved by forming complexes from these impurities as chelating agents.
Generally, the flue gas generated during the post-combustion collection process contains about 2,000 ppm of sulfur dioxide. The FGD process used to remove such a high concentration of sulfur dioxide includes wet FGD, semidry FGD, dry FGD, ammonium FGD and the like. Of these, the FGD process, which is the most widely used due to economic efficiency and high desulfurization efficiency, is a wet-limestone FGD process using limestone {Srivastava, R. K., Jozewicz, W., “Flue Gas Desulfurization: The State of the Art”, J. Air Waste Manag. Assoc. 51, 1676-1688(2001)}. Since a general wet-limestone FGD process has a desulfurization efficiency of about 90%, the concentration of sulfur dioxide after the FGD process is about 200 ppm. However, in order for the amine-based adsorbent to operate for a longer period of time, it is preferable that the sulfur dioxide concentration is further decreased to several tens of ppm and, for this purpose, additional desulfurization facilities such as sulfur dioxide filters are required. Therefore, for the process design in consideration of economic efficiency, there is a need to develop carbon dioxide adsorbents having resistance against inactivation by sulfur dioxide. The present inventors completed the present invention based on the finding that resistance to sulfur dioxide can be remarkably improved by introducing a tertiary amine-rich sulfur dioxide-resistant layer in the edge so that the adsorption selectivity for carbon dioxide is excellent and the regeneration stability is excellent in the process including sulfur dioxide.